1. Field of the Invention
The present invention relates to a semiconductor laser module used in the field of optical communication technology and a method of assembling the same.
2. Description of the Related Art
The optical communication system has grown with merits of having a large capacity and achieving a long distance transmission. As for the improved capacity of the optical communication system, information transmission of up to 10 Gbits has been made into practical use by improving the speed for modulating the light signal.
As for the increased transmission distance of the optical communication system, the transmission distance which can be achieved without a relay exceeds 100 km by combining an optical fiber in which losses of the optical transmission is small with an erbium-doped optical fiber amplifier (in the followings, referred to as EDFA).
However, the EDFA is formed with a large number of optical components such as an excitation LD module, an Er optical fiber, an optical isolator and the like. Thus, the size becomes too large so that it is unsuitable for achieving size reduction.
Accordingly, in order to meet the demands for size reduction on market, a semiconductor optical amplifier (in the followings, referred to as SOA) has been actively developed and manufactured in place for the EDFA. Compared to the EDFA, the SOA is advantageous that it is small in size, requires a low manufacturing cost and a low consumption of electricity, and so on.
A case of using the SOA will be described by referring to FIG. 1.
An optical transmission module 59 shown in FIG. 1 is used in an optical communication system with an optical transmission capacity of 10 Gbits. The optical transmission module 59 comprises a laser module 80 with a built-in optical modulator and an SOA module 60 with a built-in optical amplifier. The laser module 80 and the SOA module 60 are installed to respective packages 89 and 72, and both are connected by optical fibers 88 and 61.
As for the laser module 80, a distributed feedback (in the followings, referred to as DFB) laser element 81, a lens 82, a U-shape holder (not shown), an optical isolator 83, an element carrier 84, a Peltier device 85, a lens 86 and the like are installed inside one package 89. Further, the laser module 80 comprises a ferrule 87 and an optical fiber 88 on the output side.
The element carrier 84 is installed inside the package 89 through the Peltier element 85. The DFB laser element 81 and the optical isolator 83 are directly mounted on the element carrier 84. The lens 82 is supported by a frame 82a made of alloy and is mounted on the element carrier 84 through the frame 82a. The DFB laser element 81, the lens 82 and the optical isolator 83, with the optical axes being coincided, are mounted on the element carrier 84.
The DFB laser 81 is built in one chip integrally with the optical modulator. The light from the DFB laser element 81 is modulated in the optical modulator and the modulated light signal (signal light) is outputted. The lens 82 concentrates the signal light outputted from the DFB laser element 81 onto the optical isolator 83. The optical isolator 83 outputs the laser light from the DFB laser element 81 to the optical fiber 88 side and prevents the return light from the optical fiber 88 to the laser element 81.
The Peltier element 85 maintains the DFB laser element 81 at a constant temperature so as to stably output the modulated light signal from the optical fiber 88. The optical fiber 88 guides the signal light outputted from the optical isolator 83 to the outside of the package 89.
On the other hand, an SOA module 60, an optical fiber 61, a ferrule 62, a lens 63, a lens 64, an SOA element 65, a lens 66, a lens 67, a ferrule 68, an optical fiber 69, a carrier 70, and a Peltier element 71 are installed in a package 72.
In the top end portion of the optical fiber 61, a splice section 73 being connected to the optical fiber 88 is formed. Through the splice section 73, it is connected to the optical fiber 88 of the laser module 80 and guides the signal light outputted from the laser module 80 to the SOA module 60. The ferrule 62 fixes the optical fiber 61 to the package 72. The signal light outputted from the optical fiber 61 is guided to the lens 63. The lens 64 concentrates the signal light which has passed through the lens 63 onto the SOA element 65. The SOA element 65 operates based on the same principles as that of the semiconductor laser, in which optical amplification effect is excited on the incoming light from the outside by utilizing the gain function of the semiconductor active regions by a supply of the electric current. The lens 66 concentrates the signal light amplified by the SOA element 65 onto the lens 67. The ferrule 68 fixes the optical fiber 69 to the package 72. The signal light outputted from the optical fiber 69 is guided to the lens 67. The optical fiber 69 guides the signal light amplified by the SOA element 65 to the outside the package 72.
In order to be used for the long-distance optical communication, the signal light outputted from the DFB laser element 81 alone is not sufficient. Therefore, in the optical transmission module 59, the signal light from the DFB laser element 81 is amplified by supplying a current in the SOA element 65 to be outputted as the high-output light from the optical fiber 69.
As described above, the laser module 80 and the SOA module 60 are separately formed by being installed into respective packages. The optical fiber 88 of the laser module 80 and the optical fiber 61 on the input side of the SOA module 60 are unified by fusion-splicing to be used.
Further, recently, more high-speed and large-capacity optical communication system has been in advance. Accordingly, even higher reliability is required in the optical communication modules such as the laser module and the SOA module used for the optical communication system. As a means for achieving the high reliability, in general, widely used is a method in which the optical elements such as the optical lens and the optical fiber, which constitute the optical communication module, are fixed by laser welding (YAG weld fixing).
A conventional laser module disclosed in Japanese Unexamined Patent Publication No. 2000-208869 has a configuration as shown in FIG. 2.
As shown in FIG. 2, a lens 82 is mounted to an element carrier 84 using a U-shape holder 90.
First, weld parts 94a, 95a of the U-shape holder 90 are laser-welded to an element carrier 84 so as to mount the U-shape holder 90 onto the element carrier 84. Subsequently, a frame 82a of the lens 82 is laser-welded to weld parts 96a, 97a of the U-shape holder 90 so as to fix the lens 82 to the U-shape holder 90. At last, weld parts 98a, 99a of an optical isolator 83 are laser welded to the element carrier 84 so as to mount the optical isolator 83 onto the element carrier 84. In FIG. 2, weld parts (not shown) which are symmetrical with the weld parts 94a to 99a are provided on the opposite side which are not recognized in the figure.
On a Peltier element 85, the element carrier 84 to which a DFB laser element 81 is mounted is disposed. In order to effectively guide the optical output of the DFB laser element 81 to the optical fiber 88, it is necessary to precisely adjust the lens 82 in the emitting position of the DFB laser element 81 to be fixed to the element carrier 84. For this, the U-shape holder 90 is widely used.
FIG. 3 and FIG. 4 show a conventional U-shape holder 90 disclosed in Japanese Unexamined Patent Publication No. 07-140361.
As shown in FIG. 3 and FIG. 4, in the U-shape holder 90, a pair of opposing holding sections 92a, 92b are formed by projecting on the surface of a plate-type pedestal 91 so as to hold the lens 82 by the inner side surface of the holding sections 92a, 92b. At the same time, weld parts 94a, 95a provided on the outer side of the holding sections 92a, 92b are laser-welded. Thereby, the rear side of the pedestal 91 is fixed to the element carrier 84.
Next, described is a method of adjusting the optical axis of the lens 82 and mounting the lens 82 onto the element carrier 84. First, the frame 82a of the lens 82 is inserted to the U-shape holder 90 (FIG. 4A). By guiding the holder 90, the lens 82 is positioned in front of the DFB laser element 81 so as to dispose the lens 82 on the element carrier 84. Next, the optimum position of the lens 82 in the three axes directions is adjusted while making the DFB laser element 81 emit the light.
Specifically, as shown in FIG. 4A, the holder 90 is moved on the element carrier 84 in the direction (X direction) orthogonal to the optical axis so as to adjust the lens 82 in the optimum position with respect to the DFB laser element 81. Then, the holder 90 is laser-welded to the element carrier 84.
Subsequently, with the holder 90 being a guide, as shown in FIG. 4A, the lens 82 is moved in the vertical direction of the holder 90 so as to adjust the lens 82 in the optimum position in the Y direction with respect to the DFB laser element 81. Then, as shown in FIG. 4B, the lens 82 is moved in the direction along the optical axis (lateral direction in FIG. 4B) so as to adjust the lens 82 in the optimum position in the Z direction with respect to the DFB laser element 81. After completing the position adjustments, the frame 82a of the lens 82 is laser-welded to the holding sections 92a, 92b of the holder 90.
However, in a conventional optical transmission module, the laser module and the SOA module are installed in separate packages. Thus, there is a limit in reducing the size. Accordingly, it is inevitable that the size of the optical communication module becomes large. Also, the optical communication module is formed with two module components so that it takes time to set them up.
There are following shortcomings in the conventional U-shape holder 90. The shortcomings are emphasized in FIG. 5 and they are to be described by referring thereto.
As shown in FIG. 5, the bottom ends of the holding sections 92a, 92b of the holder 90 are welded to the element carrier 84, thereby forming the weld parts (95a, 95b) between the bottom ends of the holding sections 92a, 92b and the element carrier 84. In these weld parts (95a, 95b), a welding stress as shown by an arrow is generated when being shrunk after the welding. The welding stress is generated when the inner structures of the holder 90 and the element carrier 84 are pulled towards the weld parts (95a, 95b) sides. In this case, the mass of the element carrier 84 is sufficiently large as compared to that of the holder 90 so that it is hard to be affected by the welding stress. On the other hand, when there is the welding stress generated in the bottom ends of the holding sections 92a, 92b, as shown in FIG. 5, the opposing holding sections 92a, 92b are deformed curving towards the outer side being away from each other due to the welding stress since the holder 90 is formed in a forked shape.
When adjusting the lens 82 to be in the optimum position in the Y direction and Z direction as shown in FIG. 4A and FIG. 4B, the two holding sections 92a, 92b as the reference are deformed by the welding stress as shown in FIG. 5. Thus, it is impossible to adjust the lens 82 to be in the optimum position in the Y direction and Z direction.
Further, when the holding sections 92a, 92b are deformed by receiving the welding stress, the top ends of the two holding sections 92a, 92b are deformed in the expanding direction as shown in FIG. 5 since they are formed in a forked shape.
When a frame 82a for supporting the lens 82 is welded to the two holding sections 92a, 92b, a clearance 96 is formed between the frame 82a and the holding sections 92a, 92b. Due to the clearance 96, when welding the frame 82a for supporting the lens 82 to the holding sections 92a, 92b, weld part 97b for welding therewith is formed inside the clearances 96. Thus, the frame 82a cannot be successfully welded to the holding sections 92a, 92b, which causes a trouble when determining the position of the lens 82.